1. Field of the Invention
The present invention relates to an optical information recording medium.
b 2. Description of Background Information
FIG. 1 illustrates a cross sectional view of a conventional RO (Read Only) type optical disk and shows the manner of irradiation of a read light beam on the optical disk.
In this diagram an optical disk 101 comprises a transparent plastic substrate 102 formed of, for example, a polycarbonate resin, with one main surface of the substrate 102 serving as an information recording surface. On this information recording surface is formed a reflecting film 104 which reflects incident read light coming from an objective lens 103. The reflecting film 104, formed by vapor deposition of aluminum, for example, is sandwiched between the substrate 102 and a protective layer 105.
The information recording surface of the conventional so-called optical disk, like the reflecting film 104, has a reflection characteristic as shown in, for example, FIG. 2, which responds to incident read light of a relatively wide range of wavelengths. FIG. 2 illustrates a wavelength-dependent characteristic of a reflectance in the case of aluminum (Al) used for the reflecting film, wherein the horizontal scale .lambda.1 (nm) represents the wavelength of incident light and the vertical scale I (%) the reflectance to the incident light.
When recorded information is reproduced from an optical disk designed for long wavelength incident light, and the reproducing apparatus which uses incident read light of a short wavelength (which is used for the purpose of achieving a higher density), the read signal level will drop and intermodulation will occur, making it difficult to realize good signal reproduction. Due to high density signal recording, a disk having information recorded thereon by incident read light having a specific wavelength as a standard can only be played by the players of such a standard, ensuring no disk compatibility.